It may of had some type of film on it when you calibrated it. The film could have came off after a few hours once it was put back in the tank. When I clean mine it takes several day before I can calibrate it. The reading keeps climbing. Keep and eye on it and see if it continues to climb. Once it is stable try and calibrate it again. I have had the probe I got as a beta unit years ago and it is still working. What are your raw data values at the cal points?